1. Field of the Invention
This invention relates to operating systems for computers and computer complexes. More particularly, this invention describes a mechanism and process for sharing data among different users of the computer or computer complex.
2. Background Art
Data sharing in a multiprocessor complex, or sysplex, is accomplished in a variety of ways depending on the location of the data to be shared, (e.g. main storage, or DASD) the nature of the access permitted (e.g., read-only; read-write; etc.) and the granularity of sharing (e.g., device level sharing, data set level sharing, or record level sharing). Common to virtually all sharing mechanisms is a locking mechanism, to insure that data being modified by one user is not referenced by another user until the modification is complete. An example of a locking mechanism used in the IBM MVS system is the RESERVE mechanism for serializing access to DASD data sets. As described in MVS/ESA SPL: Application Development Macro Reference (GC28-1857-1), RESERVE permits the reservation of a device for use by a particular task on a particular system. Using this mechanism, all data sets on a RESERVEd device will be unavailable to other tasks on other systems until the owning task releases the device. While insuring data integrity, the granularity of this mechanism is clearly not fine. Another mechanism, ENQ/DEQ (described in the same publication), allows a user to define and similarly control a serially reusable resource (i.e., a resource that can be shared among users, though only one can access it at a time). Still another scheme is described in IBM Technical Disclosure Bulletin, Vol. 22, No. 6, Nov. 1979, at pp. 2571-2573: this mechanism provides for record-level sharing of a data-set across different systems by means of the storing of a user-unique key, along with a time-of-day indicator, in an access record--to serve as a lock indicator to subsequent accessors.
Common to these and most similar schemes is a deficiency in that a resource locked by a user can become lost to other users if the system of the locking user becomes disabled for an extended period of time. Further, they provide no facilities or assistance in the event of damage to the data, nor do they deal with the situation where a backup data set is to be maintained for availability purposes.